Q. CsBr crystallises in a body centred cubic lattice. The unit cell length is 436.6 pm. Given that the atomic mass of Cs = 133 and that of Br = 80 amu and Avogadro number being 6.02 × 10²³ mol⁻¹, the density of CsBr is :
A) 42.5 g/cm³
B) 2.25 g/cm³
C) 0.225 g/cm³
D) 4.25 g/cm³
Explanation
[Solution from source] CsBr has the CsCl-type structure: one Cs⁺ at the body centre and 8 × ⅛ = 1 Br⁻ at the corners, so Z = 1 formula unit. ρ = ZM/(N_A a³) = 213/(6.02 × 10²³ × (4.366 × 10⁻⁸ cm)³) = 213/(6.02 × 10²³ × 8.32 × 10⁻²³) ≈ 4.25 g/cm³. Taking Z = 2 for a bcc lattice would double this to 8.5 g/cm³. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.